The invention relates to a method for limiting an electric current through an electrical component, in particular an electrical winding, and a limiting apparatus by means of which an electric current through an electrical component can be limited.
Johncock (U.S. Pat. No. 5,321,308) is based on control of the field current through the rotor winding of a generator. The temperature of the rotor is calculated with the aid of the field current and the electrical resistance of the rotor winding. In this case, a known resistance/temperature relationship for copper is used as the basis. The field current is reduced if the rotor overheats.
Kohl, et al. (U.S. Pat. No. 5,198,744) describes a generator, in particular a starter for a motor vehicle. A field current through a field winding of the generator is controlled as a function of a measured temperature in the generator. Temperatures at specific points in the generator are preferably calculated from the measured temperature. The use of the generator temperature to control the field current allows the generator to be operated in a state where it is overexcited at times, or in high ambient temperature.
A generator, in particular for supplying power in a motor vehicle, is known from German Published, Non-Prosecuted Patent Application DE 41 41 837 A1, in which a field current through a field winding of the generator is likewise controlled as a function of a temperature, to be precise on or in a voltage regulator. The invention in this case envisages that, when a critical temperature value is exceeded, any further temperature rise owing to an excessively high field current is prevented. To this end, the field current is reduced in a suitable manner.
Busick et al. (U.S. Pat. No. 5,373,205) discloses a mathematical model for determining the temperature of an electronic switching component, for example a transistor, for engine or motor control. The temperature model is based on an exponential time function. The temperature is calculated periodically, using the model, with the calculated temperature values being compared with a maximum permissible temperature value. The calculated temperature value is in this case a function of both the instantaneous current through the component and the current in the previous period.
It is accordingly an object of the invention to provide a method for limiting an electric current through an electrical component, and a limiting apparatus that overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and that limits an electric current through an electrical component, in which overheating of the electrical component is reliably avoided but in which, at the same time, a sufficiently high electric current can be passed through the electrical component.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for limiting an electric current through an electrical component. The first step of the method is determining a time temperature profile for an electrical component. The next step is calculating a thermal load for the electrical component from the time temperature profile. The next step is limiting an electric current through the electrical component by maintaining the thermal load below a predetermined load maximum value.
With the objects of the invention in view, there is also provided a limiting apparatus for limiting an electric current through an electrical component. The electrical component receives a current and has a temperature. The integration unit integrates a time profile of the component temperature to determine a thermal load of the electrical component. The thermal load measures a material stress on the electrical component resulting from sustained high temperatures. A limiting unit connected to the integration unit limiting the current as a function of the thermal load outputted by the integration unit.
In accordance with another feature of the invention, a limiting apparatus for an electric current through an electrical component is to be provided that achieves a high exhaustion level of the magnitude of the electric current, with high operational reliability at the same time.
In accordance with another feature of the invention, a method provides for limiting an electric current through an electrical component in which a time temperature profile is determined for the electrical component and a thermal load for the electrical component is obtained from this, with the electric current being limited such that the thermal load remains below a load maximum value which can be predetermined.
The thermal load on the electrical component is a measure of the material stress in the component arising from high temperatures being present over a period of time. Because the electric current is limited on the basis of the thermal load, this on the one hand indicates that the electrical component is not thermally loaded beyond a permissible extent. On the other hand, the electric current is fully exhausted in terms of its magnitude and duration, since, using the thermal load, the magnitude and direction of the electric current can be set to be sufficiently high and for sufficiently long that there is just no longer any danger to the electrical component. In other words, the electric current can assume a maximum value and/or can be applied for a maximum maintenance period while reliably avoiding damage caused by thermal overloading. Control just on the basis of the temperature of the electrical component does not guarantee that the electric current is completely exhausted because the thermal load on the electrical component resulting from short-term high temperatures needs to be assessed differently to the load from temperatures which are raised over a lengthy time period.
A temperature limit for the electrical component is preferably defined on the basis that the electrical component is damaged after a certain time above the temperature limit, with the thermal load being calculated by summation or integration of the proportion of the time temperature profile in which the temperature is above the temperature limit.
The temperature limit is that temperature above which thermal damage will occur to the electrical component after a time that is significant in the scale of the average life. In other words, the definition of the temperature limit provides a parameter above which a significant thermal load occurs on the electrical component. The thermal load is obtained by summation and/or by integration of the temperatures in that time interval or in those time intervals in which the temperature is above the temperature limit.
The thermal load is preferably calculated using a thermal time constant of the electrical component, which thermal time constant indicates a characteristic warming-up or cooling-down time for the electrical component. The thermal time constant is used to take account of the thermal inertia of the electrical component in the calculation of the thermal load. If, for example, the electric current is switched off at a temperature above the temperature limit, then this results in the temperature of the electrical component decayingxe2x80x94generally exponentially. Despite the electric current being switched off, the temperature of the electrical component will thus still be above the temperature limit for a certain period of time. This results in a thermal load on the electrical component, which is used for controlling the limiting of an electric current that is connected once again.
The thermal load is preferably obtained from the following formula:             b      ⁡              (                  t          0                )              =                            1          A                ⁢                              ∫            0                          t              0                                ⁢                      T            ⁡                          (              t              )                                          -                        T          G                ⁢                  xe2x80x83                ⁢                  ⅆ          t                      ,
where
b(t0): is the thermal load over the time t0,
T(t): is the temperature of the electrical component as a function of time,
TG: is the temperature limit, and
A: is an integration time constant.
The integration constant A reflects the thermal inertia of the electrical component. It is preferably calculated using the ollowing formula:   A  =      Z    ⁡          [                        (                                    T              S                        -                          T              G                                )                ⁢        ln        ⁢                  xe2x80x83                ⁢                                            T              S                        -                          T              G                                                          T              S                        -                          T              u                                          ]      
where
Z: is the thermal time constant of the electrical component,
TS: is the electrical component temperature which occurs with a steady-state current, and
Tu: is the instantaneous temperature of the electrical component at which the electric current is switched off.
The time temperature profile is preferably measured or calculated. The time temperature profile for the temperature of the electrical component may be measured continuously or else in a discrete sequence using a suitable measurement apparatus. The time temperature profile measured in this way is then used to determine the thermal load. However, the time temperature profile also can be calculated. For this purpose, the temperature of the electrical component is preferably calculated as a function of the electric current, with the time temperature profile being calculated from this temperature relationship using the time profile of the electric current. The temperature of the electrical component can be calculated as a function of the electric current by taking account of the electrical resistance of the electrical component. The thermal resistance of the electrical component is preferably also taken into account. Furthermore, the temperature calculation preferably includes non-electrical losses, for example friction losses, and/or a coolant temperature of a cooling fluid that cools the electrical component.
The component temperature is preferably calculated as a function of the electric current using the following formula:       T    =                                        1                                          T                1                            -                                                R                  T                                ⁢                                                      R                    0                                    ⁡                                      (                                                                  20                        xc2x0                                            ⁢                                              xe2x80x83                                            ⁢                                              C                        .                                                              )                                                  xc3x97                                  I                  2                                                              [          "AutoRightMatch"                ⁢                              T            1                    ⁡                      (                                          T                K                            +                              T                R                                      )                              -                        R          T                ⁢                              R            0                    ⁡                      (                                          20                xc2x0                            ⁢                              xe2x80x83                            ⁢                              C                .                                      )                          ⁢                              I            2                    ⁡                      (                                          T                2                            +                                                (                                      1                    -                    x                                    )                                ⁢                                  (                                                            T                      K                                        +                                          T                      R                                                        )                                                      )                                ]
where:
T: is the component temperature;
RT: is the thermal resistance;
R0 (20xc2x0 C.): is the electrical resistance at 20xc2x0 C.;
X: is a weighting factor for the mean component temperature;
I: is the electric current through the component;
TK: is the coolant temperature;
TR: is the temperature increase due to non-electrical losses; and
T1, T2: are constants, preferably T1=255xc2x0 C., T2=235xc2x0 C.
Calculation of the temperature of the electrical component saves the instrumentation complexity for measuring the temperature of the electrical component. For example, this complexity is considerable for a rotor winding of a generator since the measured value of the temperature must be found out from the rotor while it is rotating.
The electric current through the rotor winding and/or stator winding of a generator, in particular a turbogenerator having a rating of more than 10 MVA, and preferably more than 100 MVA, is preferably limited. Owing to the very high volt-ampere densities in a turbogenerator, very high temperatures can occur in its electrical windings that, in some circumstances, cause considerable damage to the winding. The particularly stringent requirements for operational reliability of a generator necessitate a particularly reliable method for limiting the electric current through an electrical winding of the generator, and necessitate particularly accurate indications of the temperature of this electrical winding. This is ensured in a particularly reliable and nevertheless simple manner by limiting the electric current by determining the thermal load on the electrical winding.
The electric current is preferably a field current through the rotor. The field current is increased suddenly to an additional required value in a way that is referred to as xe2x80x9cfield forcingxe2x80x9d. Short-term demand peaks can occur on the volt-amperes emitted from a generator. Such a demand peak results in the power supply system voltage being dragged down. This is particularly true in the case of a generator connected to a power supply system. Increasing the field current through the rotor compensates for the voltage drag down. This increase increases the magnetic excitation field from the rotor. As a result, a higher voltage is induced in the stator. This short-term sudden increase in the field current is referred to as field forcing. The field current is increased to a specific additional demand value in a short time. This increased current generally leads to the electrical winding being heated above its temperature limit, and thus to a thermal load on the electrical winding. In the past, the increase in the field current has normally been limited to a specific time window, that is to say a predetermined time period. Once this time window had elapsed, the field current had to be limited to its rated value. This results in two problems.
Firstly, the time window may be too short. That is, depending on the thermal load on the electrical winding prior to the field forcing, the increased field current also could be maintained at the additional demand value for a longer time. This would make it possible to satisfy the increased power supply system demand better.
Secondly, if power supply system voltage fluctuations occur shortly after one another, this can lead to successive field forcings. In this case, the field current would be once again raised to the additional demand value by a second field forcing, immediately following the first field forcing, directly after it has been limited to the rated value. Thus, despite the time window provided, a number of successive field forcings can lead to an unacceptably high thermal load on the electrical winding.
These disadvantages are avoided by limiting the field current by using the thermal load on the electrical winding as a control factor. Determination of the thermal load makes it possible to identify whether the field current can still be maintained at the additional demand value for a longer time, or whether field forcing limiting must be carried out. The field current is in this case preferably maintained at the additional demand value for a maintenance period, with the maintenance period being determined on the basis of the thermal load. A first field forcing is preferably carried out, with a second field forcing following the first field forcing being allowed only if the temperature of the rotor winding is below the temperature limit. A minimum time period is preferably provided between two successive field forcings, and is complied with in all cases.
A cooling fluid temperature of a cooling fluid for the component is preferably measured, and the component temperature of the electrical component is calculated by means of the electric current and the cooling fluid temperature, with the electric current being limited such that the component temperature does not exceed a maximum value which can be predetermined.
The cooling fluid temperature is used to obtain information about the thermal load on the electrical component. The component temperature can now be calculated reliably on the basis of the electric current and the cooling fluid temperature. Such a calculation saves the hardware complexity for measuring a component temperature. This hardware complexity is considerable, especially for rotating electrical windings of an electrical rotating machine.
In the case of an electrical component that is an electrical winding of an electrical rotating machine and, in particular, is a rotor winding of a turbogenerator, the component temperature is preferably calculated while accounting for the thermal resistance of the component, the electrical resistance of the component, and the non-electrical losses. In a hydrogen-cooled turbogenerator, the hydrogen pressure is preferably taken into account in the calculation.
According to the invention, the object relating to a limiting apparatus is achieved by a limiting apparatus for an electric current through an electrical component. The electrical component has an integration unit for integration or summation of a time profile of a component temperature. In addition, the electrical component has a limiting unit, which is connected to the integration unit, for limiting the current as a function of an output signal from the integration unit.
The advantages of such a limiting apparatus lead, on the basis of the above statements, to the advantages of a method for limiting electric current.
The limiting apparatus is preferably used to limit the field current in a rotor or a turbogenerator.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for limiting an electric current through an electrical component, and a limiting apparatus, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.